Sensors for the detection of particular compounds present in a high temperature gas stream find numerous applications in many different mechanical systems. For example, detection of certain compounds such as sulfur dioxide (SO.sub.2), carbon monoxide (CO) and oxides of nitrogen (NO.sub.x), in a high temperature gas stream is important in a variety of applications.
In automotive applications, gas sensors can be placed at various locations in an exhaust system. Exhaust gas from an internal combustion engine typically contains hydrogen (H.sub.2), carbon monoxide (CO), methane (CH.sub.4), carbon dioxide (CO.sub.2), oxides of nitrogen (NO.sub.x), water (H.sub.2 O), and non-methane hydrocarbons (C.sub.n H.sub.m), where n is an integer larger than 1 and m is an integer whose value depends upon the specific hydrocarbon compound, for example, alkane, alkene or aromatics. Important environmental pollution concerns dictate that the emission of hydrocarbons be minimized. To this end, catalytic converters, which convert polluting gas species such as hydrocarbon to non-polluting gas species such as carbon dioxide and water, have been incorporated into the exhaust systems of automotive vehicles to minimize pollutants from the engine exhaust. Since these converters have a finite life, legislation has been recently proposed that would require system diagnostics that evaluate the efficiency of such converters. In this regard, sensors can be placed before and after the catalytic converter to monitor the performance of the converter. Also, the emission of hydrocarbons can be controlled, in part, by an engine exhaust control system that receives a feedback signal from an exhaust sensor capable of selectively detecting the presence of certain chemical compounds in the engine exhaust.
One method for monitoring the performance of a catalytic converter includes the use of oxygen sensors within the exhaust gas system. By measuring the amount of oxygen in the exhaust gas entering and exiting a catalytic converter, an estimate of the oxygen storage capacity of the catalytic converter can be made. As the converter ages, the oxygen storing materials within the converter sinter and lose the ability to effectively store oxygen. It was commonly believed that the catalytic materials age at about the same rate as the oxygen storing materials. As the catalytic materials age the efficiency of the converter declines. Accordingly, in theory, by estimating the oxygen storage capacity of the catalytic converter, an indirect measurement of the catalytic converter efficiency can be obtained. It has been more recently shown that this method provides a rather imprecise measure of converter performance.
A sensor that directly estimates the hydrocarbon concentration in an exhaust gas stream can be used to provide a more precise determination of catalytic converter efficiency. For example, several types of sensing elements have been developed for detecting various chemical species within an exhaust gas stream. These sensing elements include calorimetric sensors having a catalyst coating, semiconductor metal oxide based sensors, and the like. Calorimetric hydrocarbon gas sensors measure the amount of heat released by the catalytic oxidation of hydrocarbons contained within the exhaust gas.
U.S. Pat. No. 5,157,204 to Brown et al. issued Oct. 20, 1992, discloses a platinum-containing catalyst for use in removing carbon monoxide and free oxygen from hydrocarbon-containing streams by contacting the stream with at least one platinum-containing catalyst composition at a reaction temperature in the range of about -30.degree. C. to about 200.degree. C. The catalyst compositions are said to consist essentially of platinum metal, iron oxides and an inorganic support including, among others, titania, alumina, zirconia and vanadia. Other catalyst compositions are said to consist essentially of platinum metal, palladium metal, at least one manganese compound and a tin-dioxide coated ceramic carrier. Another class of catalyst compositions is said to consist essentially of platinum metal, palladium metal, at least one manganese compound, at least one chromium compound and the tin-dioxide coated ceramic carrier.
U.S. Pat. No. 4,604,275 to Murib, issued Aug. 5, 1986, discloses a method for selectively oxidizing carbon monoxide in a hydrocarbon stream employing a supported catalyst containing cobalt oxide. The catalyst is prepared by impregnating an alumina support with an aqueous solution of a water-soluble alkaline compound whose anion is capable of forming a water-insoluble cobalt compound upon reaction with a water-soluble cobalt compound.
To obtain optimum sensitivity for the measurement of hydrocarbon species within an exhaust gas, a calorimetric hydrocarbon gas sensor must be designed to maintain a relatively constant internal temperature. This requirement is especially important given the wide temperature variations encountered in an exhaust gas system. While providing a measurement of hydrocarbon concentration, a calorimetric hydrocarbon gas sensor must be carefully designed for operation in a high temperature exhaust gas stream. For precise measurement of hydrocarbons in an exhaust gas, small temperature rises or small quantities of liberated heat must be detected when the hydrocarbons are oxidized within the sensor. Detection of these small variations can be difficult when exhaust gas temperatures are rapidly changing and subjecting the sensor to a variable temperature environment. For example, automotive engine operation is dynamic and the exhaust gas temperature varies from ambient temperature, at engine start-up to more than 1,000.degree. C. during periods of high power operation. Thus, in calorimetric hydrocarbon gas sensor technology for applications to high temperature exhaust gas systems, a major technical challenge involves thermal management within the gas sensor.
Moreover, to obtain optimum sensitivity for the measurement of hydrocarbon species within exhaust gases, a catalytic gas sensor must have an adequate supply of oxidant to react with the relevant hydrocarbon compounds. This requirement is particularly important in exhaust gas applications which are frequently oxidant deficient.
In addition to the need to accommodate thermal variations within the exhaust gas, calorimetric sensors may require an additional source of oxidant for the catalytic oxidation of hydrocarbons. Typically, the oxidant supply system used in calorimetric hydrocarbon gas sensors must operate at elevated temperatures. High temperature operation is necessary to attain the level of efficiency needed to supply sufficient oxidant to the catalyst within the sensor. The necessity of including an oxidant supply system adds additional design constraints for a calorimetric hydrocarbon gas sensor.
Thus, improved catalytic materials, thermal management and an efficient oxidant supply system are needed within a differential calorimetric hydrocarbon gas sensor designed for the measurement of hydrocarbons in an exhaust gas stream.